Reversion of S plus L minus Moloney-3T3FL cells resulted from either transcriptional or post-transcriptional control of a part or whole viral genome. The selective transcription of M-MSV genome which was observed in revertants occurred by the former mechanism as evidenced by MSC-specific nucleotide sequences being lost from cellular RNA but common sequences of M-MSV being observed. The host range of MSV-Z is SC-1, NRK, and Mus caroli cells. It appears that MSV-Z is a variant of M-MSV and may be defective for integration site(s). The transcription of MSV-Z in S plus L minus SC-1 cells was restricted to various degrees. Infection of Mus caroli cells by MSV-Z was interfered with by xenotropic MuLVs, but was enhanced by Moloney or amphotropic MuLVs.